JPH0561421A - Direct current driven plane display device - Google Patents

Abstract

PURPOSE:To obtain the DC driven plane display device which can prevent the deterioration in properties of a display liquid generated by the transfer and receipt of charges between the display liquid and electrodes by coating the surfaces of the electrodes in contact with the display liquid of the plane display device, such as electrophoretic display device, with an insulator, thereby shutting off a direct current. CONSTITUTION:The plane display device which is sealed with the liquid or solid for display of the optical characteristics changing with the direct current has insulating layers 6, 7 formed on at least one surface of the electrodes 2, 4 facing each other and is so constituted that the max. value of the voltage applied on the insulating layers 6, 7 is smaller than the product of the breakdown strength of the insulating layers 6, 7 and the film thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flat display device such as an electrophoretic display device, in which the surface of an electrode in contact with the display liquid is covered with an insulating material to block a direct current, so that the display liquid and the electrode can be separated. The present invention relates to a direct current driven flat panel display device which prevents alteration of a display liquid caused by transfer of electric charges.

[0002]

2. Description of the Related Art In a DC driven flat panel display such as an electrophoretic display, two transparent base materials 1 such as glass plates having electrodes 2 and 4 formed on opposite surfaces as shown in FIG. 3 are opposed to each other at a predetermined interval, and a display liquid 5 composed of a dispersion medium colored with a dye and pigment particles is enclosed in the space, and a DC voltage is applied to the opposing electrodes 2 and 4 so that the inside of the display liquid 5 is filled. A direct current electric field is generated to move the charged pigment particles to change the distribution of the charged pigment particles, thereby performing a desired display.

However, the application of the DC voltage not only generates an electric field inside the display liquid 5, but also the display liquid 5, the electrodes 2,
DC current is generated in the circuit connecting 4 and the power supply. By the way, the specific resistance of the display liquid 5 used in the electrophoretic display element is usually 10 ⁸ Ω · cm or more, which is a relatively high resistance value.
The direct current is a low value of several μA / cm ² or less,
The influence of this DC current on the long-term life reliability is very large.

That is, the inflow or outflow of electrons from the outside to the display liquid 5 induces the oxidation / reduction of the display liquid, and for example, the display is changed due to the alteration of the dye, or the surfactant for dispersing particles is used. This is a cause of instability peculiar to the DC drive display element such as deterioration of the electrode due to deterioration or dissolution of the electrode component in the display liquid, and a combination of these obstacles.

In the past, in an electrophoretic display device, the life extension effect was aimed at by increasing the specific resistance of the display liquid to lower the direct current, or the Japanese Patent Application No. 2-8875 proposed by the applicant of the present application. As described above, the reliability is improved in the practical period by adopting the voltage application method that avoids the superposition of the DC voltage. However, such means cannot be said to be a basic measure.
Alternatively, the types and amounts of the dyes and surfactants used are limited, and the display performance itself is deteriorated.

For example, in an AC driven display device such as a liquid crystal display device or an AC driven plasma display device, a dielectric film is formed on the surface of the electrode in order to prevent display deterioration due to the electrode and charge from flowing into the electrode as well as other purposes. Although it may be provided to prevent display deterioration, in a DC drive type display element, if a DC voltage is continuously applied to maintain display, electric charge is accumulated in the dielectric film and an electric field is effectively applied to the display medium. This method has not been adopted because it cannot be applied to the dielectric film or the dielectric film causes dielectric breakdown.

[0007]

SUMMARY OF THE INVENTION According to the present invention, the surface of an electrode in contact with a display liquid is covered with an insulating material to block a direct current so as to prevent alteration of the display liquid due to transfer of electric charge between the display liquid and the electrode. The present invention provides a DC drive flat panel display device.

Therefore, the direct-current driven flat panel display device according to the present invention is a flat panel display device in which a liquid or solid for display whose optical characteristics are changed by a direct current is sealed in a gap between electrodes arranged opposite to each other. An insulating layer is formed on at least one surface of the opposing electrode, and the maximum value of the voltage applied to the insulating layer is smaller than the product of the dielectric breakdown strength and the film thickness of the insulating layer.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an enlarged cross-sectional schematic view of a principal part of a DC drive flat panel display device constructed according to the present invention.
In the figure, the same reference numerals as those in FIG. 4 indicate the same constituent elements. In FIG. 1, electrodes 2 and 4 required for display are formed of a conductive material such as indium tin oxide on the opposing surfaces of transparent substrates 1 and 3, and lead wires (not shown) are connected to these electrodes 2 and 4. It is connected to an external power source by being led out to the outside of the transparent base materials 1 and 3.

Insulating layers 6 and 7 are separately formed according to the present invention so as to cover the electrodes 2 and 4 and a part of the lead lines. As the material of the insulating layers 6 and 7,
Insulating polymers such as polyethylene, polyimide, and tetrafluoroethylene, or insulating metal oxides such as silicon oxide, barium titanate, and titanium oxide can be used. Further, as a means for forming the insulating layers 6 and 7, a usual film forming method such as coating, printing, LB film forming, vapor deposition or sputtering can be appropriately adopted. In order to construct an electrophoretic display element as a DC drive flat panel display device, the display liquid 5 is sealed in the gap between the electrodes 2 and 4 having the insulating layers 6 and 7 as described above, and the transparent substrate is formed. It can be completed by sealing the periphery of the materials 1 and 3 with an adhesive.

When a DC voltage is applied to both electrodes 2 and 4 of the above display device, the insulating layers 6 and 7 formed on the surfaces of the electrodes 2 and 4 have an equivalent capacitance in series with the equivalent resistance of the display liquid 5. And suppresses the inflow and outflow of the direct current component from the display liquid 5 to the electrodes 2 and 4.

That is, the surfaces of the electrodes 2 and 4 are covered with the insulating layers 6 and 7 made of the high dielectric material as described above, and the film thickness thereof is made sufficiently thin so that the equivalent series capacitance is made sufficiently large.
If the time constant determined by the equivalent series capacity and the specific resistance of the display liquid 5 is made sufficiently longer than the response time of this display device, the voltage applied to the display device within this response time can be kept substantially equal to the applied voltage. Moreover, since the display liquid 5 does not come into contact with the electrodes 2 and 4, it is possible to prevent the electric charge from flowing out as described above.

Of course, it can be said that the equivalent capacitance can be set to a large value. Therefore, if operating voltage pulses of the same polarity are continuously applied,
The coating film is charged with electric charges and the voltage applied to the display device is reduced, so that the original display function cannot be maintained.

The electrophoretic display device targeted by the present invention is
The display has a complete memory function, and it is not necessary to apply the same-polarity voltage for continuing the display, and the same-polarity pulse can be suppressed as in the technique of Japanese Patent Application No. 2-8875. Therefore, the present invention can be implemented in consideration of the response speed of the display, the dielectric constant and the film thickness of the coating film composed of the insulating layers 6 and 7, and the dielectric strength of the display device. Reaction can be suppressed. The case where the above insulating layer is formed on one of the electrodes will be described below.

An insulating layer of this kind formed on the electrode surface or
A protective layer made of an insulating material is similar to an AC drive display device.
However, in the case of an insulating layer according to the invention,
Is required for the performance peculiar to the DC drive display device.
It is essentially different from the conventional one. Immediately
Then, in such an insulating layer in the AC drive display device,
For example, polymer alignment film in liquid crystal display devices, AC plasma
There is a magnesium oxide protective layer in the display device,
The former is for orientation, and the latter is for ion impact relaxation or secondary charging.
It has a function as a child supply layer, which is a direct current blocking static.
It is different from the capacitance. In addition, the above conventional protective layer is required.
Withstand voltage V _IXIs the peak value of the applied AC voltage, ε_IXTo
Dielectric constant of protective layer, d_IXIs its thickness, ε₀Display the dielectric of the liquid
Rate and d₀Is the layer thickness,And is different from the present invention.

The voltage applied to the insulating layer and the display liquid 5 when a voltage is actually applied to the display device of the present invention for display is shown in FIG.
It is obtained by finding the transient response solution in the equivalent circuit of. When a display pulse having a voltage E required for display and a pulse width t _PE is applied in a state where the initial charge is zero, the voltage V _P applied to the display liquid 5 is C ₁ which is the equivalent capacitance of the insulating layer and C ₂
Is the equivalent capacitance of the display device, R is the equivalent resistance of the display liquid 5,
r is the internal resistance of the power supply, the wiring equivalent resistance, and E and −E
Is the operating voltage pulse, It is represented by.

In the above equation, when r based on the wiring and the internal resistance of the power source is small, the first term is a transient term during stepping up from zero voltage, which is a value that can be ignored in a short time.
Therefore, the second term is applied to the display liquid 5. Where R (C
_If C ₁ is designed so that _{1 1} + C ₂ ) is sufficiently larger than the pulse width t _PE required for display, that is, the capacitance of the insulating layer is large, V _P is approximately C ₁ E / (C ₁ + C _2). ), C ₁ >> C ₂ and V _P
= E, which is the same as when there is no insulating layer. In this case, unlike the case where the insulating layer is not provided, the pigment particles in the display liquid and the electrons and ions in the solvent do not give an electric charge to the electrode, and only their spatial positions are displaced, and these are deteriorated by the redox reaction. The feature is not doing.

Similarly, the voltage V _I applied to the insulating layer can be obtained,
Its maximum value is Therefore, it is necessary to design the insulating layer constructed according to the present invention so that the withstand voltage is equal to or higher than V _I.

This equation is given by the area S of the insulating layer and the film thickness d of the layer.
₁ , the dielectric constant ε ₁ , the area S of the display liquid, the thickness d ₀ , the dielectric constant ε ₀ , and the specific resistance R ₀ are Therefore, the dielectric breakdown strength of the insulating layer × d ₁ > V _I.

For example, ε ₁ = 4 × 0.0885 × 10 ^-12 (F / cm),
An SiO ₂ insulating layer of d ₁ = 0.1 × 10 ⁻⁴ (cm) was used, and d ₀ = 50 × 10
^-4 (cm), ε ₀ = 3 × 0.0885 × 10 ^-12 (F / cm), R ₀ = 10 ¹¹ (
Ω · cm) and a response speed of 100 × 10 ⁻³ (seconds), the electrophoretic display device has a V _I of about −0.28V. On the other hand, the dielectric breakdown strength of the insulating layer material is about 30 kV / mm, and the insulating strength of the insulating layer is 3 V, which can sufficiently withstand the V _I. The various constants of the electrophoretic display device calculated above are the average constants of the devices that have been prototyped so far, but even if they are different from these, select the material in consideration of the dielectric constant of the insulating layer to be formed and its breakdown strength. can do.

Further, after applying the above-mentioned display voltage pulse,
The voltage of each part when a pause pulse of zero voltage is applied will be described. When the time after the applied voltage is set to zero is t, the voltage V _P applied to the display liquid is Although the voltage of the opposite polarity of is applied, C ₁ is set so that R (C ₁ + C ₂ ) is sufficiently larger than t _PE.
The absolute value of _P- is small and does not affect the display. Further, in order to reduce the application of the voltage of the opposite polarity to the display device, as shown in FIG. 3, if R _S larger than R is installed to suppress the charge inflow from C ₁ to C ₂ , V _P- Further reduction is possible.

In driving a display device having an insulating layer as described above, a display memory is effectively used, and the above-mentioned Japanese Patent Application No.
The purpose of this is to prevent the superposed application of the same-polarity voltage pulse as embodied in the application of No. 8875, and to prevent the excessive charging of this insulating layer. Therefore, a voltage of the opposite polarity for discharging the electric charge of the insulating layer is applied to the element which has once displayed, and the voltage applied to the insulating layer does not exceed the above-mentioned V _I.

The above description describes specific conditions of the insulating layer and the driving method in the DC driving flat panel display device when the insulating layer is formed on either one of the electrodes 2 and 4. However, it is also possible to cover the pair of electrodes facing each other with the same insulating layer, and it is also preferable from the viewpoint of improving the performance which is the object of the present invention.

Further, although the above description has been made on the electrophoretic display device as the DC driving flat panel display device, it can be applied to other DC driving flat panel display device having a memory function such as an electrochromic display device. ..

[0025]

The DC driven flat panel display device according to the present invention comprises:
As is clear from the above description, by covering the electrode surface in contact with the display liquid with an insulator to cut off the direct current, the deterioration of the display liquid caused by the transfer of the electric charge between the display liquid and the electrode,
That is, the deterioration of the display liquid due to the oxidation / reduction of the ions in the display liquid can be prevented.

Therefore, it is possible to prevent the discoloration of the dye, the deterioration of the dispersed state, the change of the electrophoretic property of the dispersed particles, and the like.

[Brief description of drawings]

FIG. 1 is a conceptual cross-sectional configuration diagram of a DC drive flat panel display device configured according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram when an insulating layer is formed on one electrode in FIG.

FIG. 3 is an equivalent circuit diagram when an external resistor is installed in FIG.

FIG. 4 is an enlarged cross-sectional configuration diagram of a conceptual main part of a conventional electrophoretic display device.

[Explanation of symbols]

 1 Transparent Base Material 2 Electrode 3 Transparent Base Material 4 Electrode 5 Display Liquid 6 Insulating Layer 7 Insulating Layer

Front page continuation (72) Takashi Mori Inventor Takashi Mori 757 Tenhoki, Kakizaki-cho, Inashiki-gun, Ibaraki Minami-Ibaraki Plant, Nippon Mektron Co., Ltd.

Claims (1)

[Claims] 1. In a flat display device in which a liquid or solid for display whose optical characteristics are changed by a direct current is enclosed in a gap between electrodes arranged facing each other, an insulation formed on at least one surface of the electrodes facing each other. A direct-current driven flat panel display device comprising a layer, wherein a maximum value of a voltage applied to the insulating layer is smaller than a product of a dielectric breakdown strength and a film thickness of the insulating layer.